Cathodically depositable paints generally have very good corrosion resistance on zinc phosphate-treated substrates. Adequate corrosion resistance on non-pretreated sheet steel, referred to as bright sheet steel, heretofore could only be achieved, however, by the addition of lead compounds in a suitable form. All lead compounds heretofore used for this purpose in cathodically depositable electrodeposition paints have major disadvantages. The lead compounds used as solid substances, such as lead oxide or basic lead silicate, can only be dissolved in the finished paint. The paints, therefore, must undergo fairly long homogenization periods until the lead salt has attained its required efficacy as a catalyst. The water-soluble salts thus formed, like other water-soluble salts added directly to the paint, extend into the aqueous phase of the paint rather than into the resin micelles. Accordingly, during electrodeposition of the paint, as a result of the endosmosis effect occurring in the film, which results in dehydrating the film, the water-soluble salts are washed out of the film or are precipitated onto the substrate as a metallic deposit. Dissolved salts are also removed to some extent from the electrodeposition bath by the dialysis equipment used in electrodeposition paint equipment and possibly recycled into the bath in uncontrolled manner during the rinsing operation.
The water-insoluble salts of longer-chained fatty acids are soluble in the resin and, therefore, remain to a large extent in the resin micelle. However, hydrolysis produces measurable quantities of water-insoluble fatty acids which considerably disturb the precipitation process and the control of the bath during electrodeposition painting.
Lead catalysts suitable for cathodic electrodeposition painting are described in Austrian Application No. 2760/86, which corresponds to U.S. Application Ser. No. 109,521 filed Oct. 19, 1987, now U.S. Pat. No. 4,788,234 issued Nov. 29, 1988. The described catalysts which are reaction products of lead(II) oxide and compounds containing .beta.-hydroxyalkylamino groups do not have the usual disadvantages as above described. However, there is an increasing tendency in industry to avoid using lead compounds in paints because of the toxicity of the compounds and the resultant disposal problems. On the other hand, however, adequate corrosion prevention of bright sheet steel is an essential requirement of users of cathodically depositable electrodeposition paints, particularly in the automobile industry.
Cathodically depositable electrodeposition binders which crosslink through transesterification and transurethanization have utilized tetraalkyl orthotitanates and/or titanium acetyl acetonates instead of the lead compounds. However, such compounds are susceptible to hydrolysis and during aging of the paint bath are responsible for increasing deficiencies in the deposited films.